The study was initiated in September 1977 with 1000 children in the seventh grade of nine junior high schools located in Des Moines, Iowa, a community that has optimal amounts of fluoride in its supply of drinking water. The children were randomly assigned to one of the following three study groups: Group I (controls) rinse their mouths once every week in school for 60 seconds with a placebo solution; Group II follows an identical procedure using a 0.2% neutral sodium fluoride solution (0.09%F); Group III rinse their mouths once every day in school for 60 seconds using a 0.05% neutral sodium fluoride solution (0.023%F). The procedures are carried out under the classroom teacher's supervision for a period of three years. Baseline dental examinations, using the DMF surface index, were conducted in November 1977. Follow-up examinations will be conducted in May 1979 and May 1980.